What is the exact error message?

I believe it says the server is rejecting my e-mail, because it is incorrect and I may need to check it in my address book, which I have already done and it is correct.....

The exact error message is needed.

Rick, it says "An error occurred while sending e-mail. The mail server responded:<psmart@triad.rr.com> sender rejected. Please see https://www.spectrum.net/support/internet/understanding-email-error-codes. For more information. AUP#Out-1320. Please verify that your email address is correct in your account settings and try again."

I know very little about computers and need step by step instructions on what to do....Thanks, Paul Smart

Step 1: Look up the error code. Here is what Spectrum says:

"Spectrum limits the number of concurrent connections from a sender, as well as the total number of connections allowed. Limits vary based on the reputation of the IP address. Reduce your number of connections and try again later."

Step 2: Call Spectrum for help. Ask them about connections and your reputation as a sender and how you can avoid these errors.

One factor in the quantity of connections is the quantity of devices you are using to access e-mail. Your reputation is probably based on factors that can indicate spamming: quantity of messages sent, whether or not messages have an opt-out or unsubscribe function, et cet.

In short, you may be sending too many messages that have similarities to spam from too many devices. In any case, it is not a Thunderbird issue.